1. Technical Field
The present invention relates to an ultrasonic transducer, an ultrasonic speaker and a method of driving and controlling the ultrasonic transducer. More specifically, the invention relates to an electrostatic ultrasonic transducer capable of outputting sonic waves to input signals, an ultrasonic speaker and a method of driving and controlling the ultrasonic transducer.
2. Related Art
Piezoelectric and electrostatic transducers are typical ultrasonic transducers. A piezoelectric transducer incorporates a piezoelectric element, such as a piezo device, as a vibration body, and is a resonance-type transducer. With this type of transducer, a resonant frequency band is utilized for driving. Therefore, a piezoelectric transducer has the feature that it can efficiently generate a high sound pressure and has a narrow band sound pressure-frequency characteristic. In contrast, an electrostatic transducer has an electrode film that is vibrated by causing an electrostatic force to act between a fixed electrode and the electrode film, and has a wide band sound pressure-frequency characteristic.
It is known that when a modulated wave (sonic wave) resulting from the amplitude modulation of an ultrasonic carrier of a high sound pressure by an acoustic signal in an audio band is directed into the air, the sound velocity is made higher at locations of a high sound pressure and lower at locations of a low sound pressure because the air has nonlinearity, distorting the waveform of the sonic wave as the sonic wave propagates in the air. This leads to the accumulation of waveform distortion to gradually attenuate components of the carrier and thus components of the acoustic signal in the audio band used in the modulation are gradually self-demodulated as the sonic wave propagates in the air. This phenomenon is referred to as parametric array. A self-demodulated audible sound has a sharp directivity due to transportation by an ultrasonic wave and as such, a speaker to which the principle is applied is referred to as e.g., a parametric speaker or ultra-directional speaker (ultrasonic speaker).
A conventional ultra-directional speaker commonly incorporates a resonance-type transducer because an ultra-directional speaker (ultrasonic speaker) needs the generation of a high sound pressure, (see e.g., JP-A-2003-47085 and JP-A-2004-112212). However, such a conventional ultra-directional speaker is often regarded as being lower in reproduction sound quality in comparison to a loudspeaker and therefore is only used for voice, e.g. a local announcement or a comment on an exhibit. As described above, a resonance-type transducer has a narrow band sound pressure-frequency characteristic and is limited in its drive frequency. As such, it has the following problems: it is difficult to enhance its reproduction sound quality; and it is hard to adjust the reproduction range. Also, there is a problem in that caution must be exercised in handling a resonance-type transducer because it is sensitive to excessive input and easy to damage an element therein.
On the other hand, an electrostatic transducer is smaller than a resonance-type transducer in output sound pressure per unit area, but has a wide band sound pressure-frequency characteristic. Therefore, an electrostatic transducer has the following features: it is easy to enhance its reproduction sound quality; and it is also easy to adjust the reproduction range. A vibration body (film) of an electrostatic transducer is more flexible than that of a resonance-type transducer. Therefore, an electrostatic transducer has the following features: it is less prone to being damaged by excessive input; and it doesn't have to be handled nervously (as cautiously) as in handling a resonance-type transducer.
Thus, from the viewpoints of the enhancement of the reproduction sound quality and the ease of handling, it is preferable to use an electrostatic transducer to form an ultra-directional speaker.
Electrostatic transducers may be roughly classified into two types, i.e., pull type and push-pull type. Their advantages and drawbacks are as follows.
FIGS. 8A and 8B are views for explaining a concept for driving a pull type electrostatic ultrasonic transducer. In the transducer, alternating current (AC) signals superimposed on a direct current (DC) bias voltage output by a DC bias source are applied between a fixed electrode 20 and a vibrating-electrode film 10 including a vibrating film (insulating film) and a conducting layer deposited on the vibrating film to vibrate the vibrating-electrode film 10 according to the AC signals, thereby outputting ultrasonic waves.
FIG. 8A shows a state of amplitude of the vibrating-electrode film 10 when a plus (+) side output of an AC signal with a direct current (DC) bias voltage superimposed thereon is applied to the vibrating-electrode film 10. FIG. 8B shows a state of amplitude of the vibrating-electrode film 10 when a minus (−) side output of the AC signal with a DC bias voltage superimposed thereon is applied to the vibrating-electrode film 10.
In the case of the state shown in FIG. 8A, the potential difference between the fixed electrode 20 and the vibrating-electrode film 10 is enlarged to cause a strong electrostatic force (attracting force) to act between the fixed electrode 20 and the vibrating-electrode film 10, whereby a center portion of the vibrating-electrode film 10 is attracted toward the fixed electrode 20. In the case of the state shown in FIG. 8B, the potential difference between the fixed electrode 20 and the vibrating-electrode film 10 is reduced to weaken the electrostatic force (attracting force) between the fixed electrode 20 and the vibrating-electrode film 10, whereby the center portion of the vibrating-electrode film 10 is drawn back in the direction opposite to the fixed electrode 20 by an elastic restoring force. In this way, the vibrating-electrode film 10 is vibrated according to AC signals to output ultrasonic waves.
Unlike a push-pull type electrostatic ultrasonic transducer (which is to be described later), a pull type electrostatic ultrasonic transducer like this doesn't require the provision of a through-hole to allow sonic waves to pass therethrough or the like in the fixed electrode. Therefore, a pull type electrostatic ultrasonic transducer is advantageous in that: its aperture ratio is large; and it is easy to secure a sound pressure. However, a pull type electrostatic ultrasonic transducer has the drawback that the distortion of its output waveform is made larger. This is because the constituents that contribute to the vibration are only an electrostatic attracting force and an elastic restoring force of the film.
FIGS. 9A-9C are views of explaining a concept for driving a push-pull type electrostatic ultrasonic transducer. In a push-pull type electrostatic ultrasonic transducer, an upside fixed electrode 20a and a downside fixed electrode 20b are provided on the opposing sides of a vibrating-electrode film 10 so as to be opposed to the film 10. A DC bias source supplies a positive side DC bias to the vibrating-electrode film 10 and then an AC signal is applied between the upside fixed electrode 20a and the downside fixed electrode 20b. 
FIG. 9A shows a state of amplitude of the vibrating-electrode film 10 when the AC signal is zero (0) volts. In this case, the vibrating-electrode film 10 is situated in its neutral position (in the middle between the upside fixed electrode 20a and the downside fixed electrode 20b).
FIG. 9B shows a state of amplitude of the vibrating-electrode film 10 when a plus voltage of the AC signal is applied to the upside fixed electrode 20a and a minus voltage of the AC signal is applied to the downside fixed electrode 20b. A center portion of the vibrating-electrode film 10 is attracted toward the downside fixed electrode 20b due to an electrostatic force (attracting force) between the film 10 and the downside fixed electrode 20b and an electrostatic force (repulsion force) between the film 10 and the upside fixed electrode 20a. 
FIG. 9C shows a state of amplitude of the vibrating-electrode film 10 when a minus voltage of the AC signal is applied to the upside fixed electrode 20a and a plus voltage of the AC signal is applied to the downside fixed electrode 20b. The center portion of the vibrating-electrode film 10 is attracted toward the upside fixed electrode 20a due to an electrostatic force (attracting force) between the film 10 and the upside fixed electrode 20a and an electrostatic force (repulsion force) between the film 10 and the downside fixed electrode 20b. 
In this way, the vibrating-electrode film 10 is vibrated according to AC signals to output sonic waves.
A push-pull type electrostatic ultrasonic transducer like this has the advantage that the distortion of its output waveform is made smaller. This is because both an electrostatic attracting force and an electrostatic repulsion force act on the vibrating film, namely plus and minus electrostatic forces act on the film symmetrically. However, a push-pull type electrostatic ultrasonic transducer outputs sonic waves through a through-hole provided in the fixed electrode and therefore it has the following drawbacks: its aperture ratio is small; and it is hard to secure sound pressure.
In the case where an electrostatic transducer is used as an ultra-directional speaker, there is the following specific problem: even if an ideal amplitude-modulated wave in an ultrasonic wave band is input to the speaker, when the transducer outputs a waveform (carrier) whose plus-and-minus asymmetric distortion is large, components of the distortion make audible sound components and the audible sounds in addition to ultrasonic wave components are to be output directly from the speaker, degrading the directivity in audibility. This is because electrostatic transducers have wide frequency band sound pressure characteristics (i.e. even when an audible sound is directly input to the transducers, a sound pressure is output in its own way). Hence, it can be said that this is a problem specific to transducers having wide frequency band characteristics. Therefore, in order to avoid such problem, it is desirable to use a push-pull type transducer which can output a waveform with a smaller distortion in comparison to a pull type one.
However, a push-pull type transducer requires the provision of a through-hole to allow sonic waves to penetrate therethrough in the fixed electrode in order to emit sounds to the outside. This poses the following problem: it is difficult to appropriately raise a sound pressure because increasing the aperture ratio reduces electrostatic force acting on the vibrating film thereby lowering sound pressure, and reversely increasing an electrostatic force per unit area lowers the aperture ratio. In addition, a push-pull type ultrasonic transducer has the following problem: it is costly because it is more difficult to manufacture in comparison to a pull type, and requires high precision in machining and positioning.
The first requirement to realize a high directional speaker is to generate a high sound pressure. A pull type transducer can realize the generation of a high sound pressure more easily in comparison to a push-pull type transducer.
Now considered is the case where a sine wave drive signal is supplied to a pull type electrostatic transducer to drive it. In the pull type electrostatic transducer, a DC bias voltage is applied between the vibrating-electrode film and the fixed electrode to cause an electrostatic attracting force to work thereby to apply a tension force to the vibrating film. In this condition, an AC signal is superimposed on the DC bias voltage to force the electrostatic attracting force to fluctuate, whereby the vibrating film is vibrated.
As described above, only an electrostatic attracting force and an elastic force (restoring force) caused by the film act on the vibrating film, and therefore the forcedly vibrating force acting on the film is just an electrostatic attracting force. Hence, it is harder to vibrate the vibrating film symmetrically in plus and minus directions (i.e. upward and downward) in comparison to a push-pull type transducer in which an attracting force and a repulsion force act on a vibrating film symmetrically from both the up and down side fixed electrodes.
FIG. 10 is a view showing an example in which a vibrating waveform is distorted asymmetrically in the up and down directions. As shown in the drawing, even when a sine wave signal (i.e. a waveform drawn by a dotted line) having plus-side and minus-side amplitudes identical in size with each other is input, the signal vibrates asymmetrically in the plus and minus directions as shown in the solid line.
Therefore, in the case where a conventional pull type transducer is used to constitute an ultra-directional speaker, there is the problem that a bilaterally asymmetrical distortion is created in its output waveform (carrier), degrading the directivity in audibility.